Field of the Invention
The present invention relates to a method of dispersing fibers in electromagnetic-attenuating coating materials.
Electromagnetic interference (EMI) shielding materials are disclosed in U.S. Pat. Nos. 4,935,296 and 5,089,326. The ""296 patent discloses a composite containing an EMI shieldingly effective amount of metal-coated fibers and details of such fibers can be found in U.S. Pat. Nos. 4,738,896; 4,759,950; 4,789,563 and 4,839,402.
Radar absorptive coating materials are disclosed in U.S. Pat. Nos. 3,599,210 and 3,843,593. The ""210 patent discloses a coating containing fibers cut to a length of one-half wavelength of the anticipated radar frequency with the fibers being insulated along their lengths by thinly bonding a dielectric, low loss material such as unfilled epoxy, pure resin or varnish and the fibers are randomly dispersed within a solid matrix of resinous material. The ""593 patent discloses a coating containing iron pigment in a resin matrix.
U.S. Pat. No. 4,983,456 discloses electrically conductive laminates, conductive coatings, conductive adhesives, conductive inks, conductive gaskets and conductive caulking and sealing compounds wherein metal coated fibers are utilized. The ""456 patent further discloses polymeric material convertible by heat or light, alone or in combination with catalysts, accelerators, cross-lining agents, etc., can be combined with the composite fibers. The fibers include a semi-metallic core made of carbon, boron or silicon carbide and an electrically and/or thermally conductive layer of at least one electro-depositable metal such as nickel, silver, zinc, copper, lead, arsenic, cadmium, tin, cobalt, gold, indium, iridium, iron, palladium, platinum, tellurium, tungsten or mixtures thereof.
The following U.S. patents relate generally to methods of mixing filler or fibers in resin matrixes and/or applying a mixture of resin and filler/fiber material onto a substrate. The patents include U.S. Pat. Nos. 2,923,934; 2,929,436; 3,599,210; 4,325,849; 4,360,440; 4,474,685; 4,534,998; 4,862,713; 4,935,296; and 4,983,456. The foregoing patents, however, fail to appreciate the importance of avoiding breakage of fibers during mixing of the fibers with a resin matrix and during applying the mixture to a substrate. For instance, with respect to electromagnetic-attenuating materials such breakage could result in fibers having lengths that reduce, and conceivably even eliminate the desired electromagnetic-attenuating property.
The invention provides a method of dispersing fibers in an electromagnetic-attenuating coating and applying the coating to a substrate. The method includes steps of mixing a coating material comprising fibers and resin in a container such that the fibers are uniformly dispersed in the resin without breaking the fibers. The method also includes feeding the coating material from the container while maintaining the fibers uniformly dispersed in the resin and without breaking the fibers. In addition, the method includes applying the coating material to a substrate.
According to various features of the invention, the mixing step can be carried out by subjecting the container to an oscillatory shaking action such that the fibers are subjected to a high-shear mixing action without breaking the fibers. The feeding step can be carried out by pressurizing the container and ejecting the coating material through an opening in the container. For instance, the coating material can be fed through a conduit attached to a bottom opening in the container. The applying step can comprise spraying the coating material on the substrate. For instance, the spraying can be carried out with an air-nozzle-spray gun.
The electromagnetic-attenuating coating can comprise fibers in a dielectric matrix material. The fibers can comprise metal-coated dielectric or semiconductive fibers. For instance, the fibers can be selected from the group consisting of glass, fused silica, silicon carbide and graphite fibers. The fibers preferably have a uniform diameter, a circular cross-section, a rectilinear shape, a smooth exterior surface and a diameter of 4-20 microns (xcexcm). For instance, the fibers can be rectilinear nickel-coated graphite fibers. The fibers are preferably uniformly dispersed in the resin and can comprise less than 1%, preferably less than 0.2% by weight of the matrix. The resin can comprise a thermosetting plastic material such as a polymer material or a synthetic resin such as silicone. The resin is preferably a non-thermoplastic material.